Solid imaging compositions for preparing polyethylene-like articles

ABSTRACT

This invention discloses compositions adapted to produce, through solid imaging means, excellent quality objects having material properties that simulate the look and feel of polyethylene articles.

FIELD OF THE INVENTION

[0001] This invention discloses compositions adapted to produce, throughsolid imaging means, excellent quality objects having materialproperties that simulate the look and feel of polyethylene articles.

BACKGROUND OF INVENTION

[0002] In the field of liquid-based solid imaging, alternatively knownas stereolithography, compositions have been developed which are capableof generating solid objects having the properties of epoxies and/oracrylates. Solid imaging generated objects made from previous epoxyand/or acrylate compositions provide a prototypical representation ofthe physical shape of plastic articles made on a production basis out ofmaterials such as ABS, nylon, polyethylene, polypropylene, etc. However,such compositions lack the material properties that give users of theprototypes a sense of look and feel for the object when produced in theproduction material. Such a lack of look and feel accuracy in productprototyping is not just an aesthetic issue. The look and feel of aprototype also has significant engineering, design, packaging, labeling,and advertising implications.

[0003] For example, squeeze bottles, such as those used for dispensingdish soap, are designed to be attractive in shape, easy to grasp, andeasy to squeeze. Typically such bottles are made from polyethylene typepolymers. Previous epoxy and/or acrylate compositions used in solidimaging were capable of producing articles that have the same attractiveshape. However, the stiffness of the prototyped articles made from thesematerials was likely to mislead the designer and evaluator of thearticle relative to such issues as, for example, wall thickness andsurface radius design. For example, a commercial solid imaging resin,Somos® 2100 (E. I. DuPont De Nemours, Inc., Wilmington, Del.), producesarticles having lower stiffness than polyethylene. Bottles made fromthis material do not provide adequate resistance to squeezing such thatenough friction occurs between a person's fingers and the bottle. Aperson holding a Somos® 2100 prototype bottle of liquid soap is likelyto squeeze too hard in order to generate enough friction to keep thebottle from slipping. As a consequence, the soap is likely to bedispensed prematurely. The designer of the bottle might then be misledinto making the wall thickness of the bottle greater in order to improveits stiffness. But such a design change would lead to a bottle that istoo stiff when manufactured with polyethylene. Similar problems aregenerated when other much stiffer epoxy and/or acrylate compositions areused to prototype articles such as bottles. A designer might be led todecrease the wall thickness of the bottle due to the stiffness. Or forexample, since sharper radii may not feel as comfortable duringsqueezing when stiffer materials are used, the designer may be misledinto re-designing the bottle with greater radii. This may affect thebottle squeezability when manufactured in polyethylene or may reduce theaesthetic appeal of the bottle shape.

[0004] Other examples may be made regarding the importance of appearanceof an article when made out of certain materials. For example, use of atransparent prototype composition or an overly opaque composition maymislead those viewing the article into incorrect assumptions regardingappropriate packaging, labeling, coloring, and advertising of a product.

[0005] Other considerations when trying to utilize solid imaging forprototyping include photospeed, resistance to humidity, low potentialfor hydrolysis, similar coefficient of friction, dimensional accuracy,ability to span without supports during fabrication, and wide processlatitude.

[0006] Japanese Patent Application Hei 2-75618 describes epoxy andacrylate compositions for use in optical molding. The compositionscontain at least 40 wt % of alicyclic epoxy resin with at least twoepoxy groups in each molecule.

[0007] U.S. Pat. No. 5,476,784 describes cationic epoxy and acrylatecompositions for use in solid imaging. The compositions may comprisefrom 5-40% by weight of at least one OH-terminated polyether, polyesteror polyurethane. In the examples given, the polyether polyolsformulations provide lower elongation at break properties that theelongation at yield properties of most low-density polyethylenes.Additionally, the patent teaches that the epoxy content is to be from 40to 80% of the formulation by weight. Compositions made with this epoxycontent are likely to produce cured articles having a higher modulusthan that of polyethylene.

SUMMARY OF INVENTION

[0008] This invention discloses photosensitive compositions comprising;

[0009] (a) about 20-40% by weight of an epoxide-containing material;

[0010] (b) about 5-40% by weight of acrylic material selected fromaromatic acrylic material, cycloaliphatic acrylic material, orcombinations thereof;

[0011] (c) about 5-50% by weight of a reactive hydroxyl-containingmaterial;

[0012] (d) at least one cationic photoinitiator; and

[0013] (e) at least one free-radical photoinitiator; with the provisothat upon exposure to actinic radiation an article is produced havingthe following properties:

[0014] (i) a tensile break before yield stress or a tensile yield stressgreater than 13 N/mm2;

[0015] (ii) a tensile modulus in the range of about 180 to about 850N/mm2;

[0016] (iii) a tensile break elongation before yield or a tensile yieldelongation greater than 6%; and

[0017] (iv) a notched Izod impact strength greater than 50 J/m.

[0018] The invention also relates to a process for forming athree-dimensional article, said process comprising the steps:

[0019] (1) coating a thin layer of a composition onto a surface;

[0020] (2) exposing said thin layer imagewise to actinic radiation toform an imaged cross-section, wherein the radiation is of sufficientintensity to cause substantial curing of the thin layer in the exposedareas;

[0021] (3) coating a thin layer of the composition onto the previouslyexposed imaged cross-section;

[0022] (4) exposing said thin layer from step (3) imagewise to actinicradiation to form an additional imaged cross-section, wherein theradiation is of sufficient intensity to cause substantial curing of thethin layer in the exposed areas and to cause adhesion to the previouslyexposed imaged cross-section;

[0023] (5) repeating steps (3) and (4) a sufficient number of times inorder to build up the three-dimensional article; with the proviso thethree-dimensional article has the following properties:

[0024] (i) a tensile break before yield stress or a tensile yield stressgreater than 13 N/mm2;

[0025] (ii) a tensile modulus in the range of about 180 to about 850N/mm2;

[0026] (iii) a tensile break elongation before yield or a tensile yieldelongation greater than 6%; and

[0027] (iv) a notched Izod impact strength greater than 50 J/m.

[0028] The invention also relates to the above process wherein thecomposition comprises:

[0029] (a) about 20-40% by weight of an epoxide-containing material;

[0030] (b) about 5-40% by weight of an aromatic or cycloaliphaticacrylic material;

[0031] (c) about 5-50% by weight of a reactive hydroxyl-containingmaterial;

[0032] (d) at least one cationic photoinitiator; and

[0033] (e) at least one free-radical photoinitiator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 depicts a stress-strain curve of a tensile or flexuralsample.

[0035] FIG. 2 depicts the effect of epoxy content on the modulus of acomposition and shows the modulus range for various grades of low tomedium density polyethylene.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Liquid based Solid Imaging is a process wherein a photoformableliquid is coated into a thin layer upon a surface and exposed imagewiseto actinic radiation such that the liquid solidifies imagewise.Subsequently, new thin layers of photoformable liquids are coated ontoprevious layers of liquid or previously solidified sections. Then thenew layer is exposed imagewise in order to solidify portions imagewiseand in order to induce adhesion between portions of the new hardenedregion and portions of the previously hardened region. Each imagewiseexposure is of a shape that relates to a pertinent cross-section of aphotohardened object such that when all the layers have been coated andall the exposures have been completed, an integral photohardened objectcan be removed from the surrounding liquid composition.

[0037] One of the most important advantages of the solid imaging processis the ability to rapidly produce actual objects that have been designedby computer aided design. A significant amount of progress has been madewith compositions and processes that have been adapted to improve theaccuracy of the objects produced. Also, composition developers have madesignificant progress toward improving individual properties such as themodulus or deflection temperature of the photohardened objects. However,attempts to simulate a particular set of physical properties of a commonmanufacturing material to such a degree that the simulation materialcould be easily mistaken for the simulated material, based upon look andfeel properties, have been unsuccessful.

[0038] During the development of the compositions disclosed herein, itwas noted that substantial changes in the look and feel of articlesfabricated by the liquid solid imaging process could be attained byslight alterations in component concentration. Surprisingly it was foundthat by making these alterations in composition, articles could be madethat had the look and feel of articles manufactured from polyethylenematerials. Within the field of liquid solid imaging such a discovery isa first in that previous commercial compositions did not make articlesthat elicited a similar look and feel sense with regard to any othercommon plastic. It was then recognized that by tailoring thecomposition, the properties of polyethylene manufactured articles couldbe simulated. This potential therefore solved an oft expressed butunfilled need to produce prototypes that not only had the appearance ofdesired objects but also material properties that simulated the look andfeel of the materials out of which production objects were destined tobe manufactured.

[0039] In order to simulate a material in terms of look and feel, it isnecessary to decide upon the appropriate appearance factors and physicalproperties. For example, in the field of liquid solid imaging the mostcommonly quoted fully cured part physical properties are the tensilestress, tensile modulus, elongation at break, flexural stress, flexuralmodulus, impact strength, hardness, and deflection temperature. Some ofthese physical properties, such as for example elongation at break, arenot something that can be “felt” unless the material is deformed. Suchphysical properties are therefore not indicative of a good simulationmaterial property.

[0040] In some cases, the characteristics of a material that serve todefine the look and feel properties of a particular material aredifficult to define. This is especially so in the case of how amaterials looks. However, in the case of the instant invention adeliberate compositional choice was made such that articles fabricatedthrough solid imaging means, when given various amounts of exposure toactinic radiation, had a similar color and light scatteringcharacteristic as various grades of polyethylene. It was also found thatchanging the actinic exposure can also modify the feel properties of thearticles manufactured from the composition by the solid imaging process.That is, when lower actinic exposures are given during the solid imagingprocess, the articles feel more like lower density polyethylene articlesbut when higher actinic exposures are given, the articles feel more likemedium density polyethylene articles.

[0041] FIG. 1 depicts a typical stress-strain curve for a tensile or aflexural sample. A stress-strain curve, also known as a deformationcurve, shows the relationship between the stress or load on a sample andthe strain or deformation that results. The tensile stress-strain curveresults when the sample is under tension. The flexural stress-straincurve is a result of bending the sample. It is considered here that thetensile properties are best representative of how the articles feel.

[0042] In FIG. 1, the stress is shown on the y-axis and the strain % isshown on the x-axis. A typical representation of a material response tostress is shown as the dark curve. At the beginning of stress, thestress-strain relationship is roughly linear. The “tensile modulus” (orthe flexural modulus) is defined as the slope of the curve in thislinear region. As the sample continues to be stressed, the stress-straincurve tends to become less linear until a maximum is reached. Thismaximum point is called the “yield point” and is generally defined asthe region where a large increment of extension occurs under constantload. The “yield stress” is the stress at the yield point. The“elongation at yield” is the percent elongation at the yield point. Somematerials exhibit increased stress and strain capability beyond theyield point. The point at which the sample breaks is where the values of“break stress” and “% elongation at break” can be ascertained. Somesamples may break prior to the yield point or at the yield point. In thecase of break before yield, only the break stress and % elongation atbreak values are reported. It is possible for the break stress to be alower or higher value than the yield stress. All tensile properties asdiscussed herein were measured according to ASTM Test D-638M, exceptthat for the example formulations and the comparative exampleformulations the humidity was not controlled.

[0043] By far, the most important property that relates to what is feltwhen handling a material, is the tensile modulus. This is representativeof the feeling of stiffness.

[0044] A second important property is that of the elongation of thematerial. When a simulation of a material is handled and flexed, itshould not break or permanently distort if the material being simulateddoes not break or distort with such handling. With plastics there isconsiderable debate relating to the point at which a sample under stresstransitions from an elastic mode to a plastic mode of behavior. However,most would agree that when a material begins to yield, its behavior isplastic and that any handling that brings a sample past its yield pointwill leave the sample permanently distorted. For the purposes of thisinvention the tensile elongation at yield serves to help define thisaspect of the feel of a material. If a material breaks before its yield,it must have a tensile elongation at break that is greater than or equalto the tensile elongation at yield of the material being simulated or itis not an acceptable simulation material. Ideally a simulation materialshould have an elongation at yield that is about the same or greaterthan the elongation at yield of a simulated material.

[0045] A third important physical property is the tensile stress. Forthe purposes of this invention, a tensile stress for a material thatbreaks at or before its yield is an important property for simulationpurposes. Simulation materials that have a yield stress or break stress(before yield) that is lower than the lowest yield stress or breakstress (before yield) of a simulated material are usually unsuitablesimulation materials.

[0046] Another important physical property is that of the sense of feelfor inherent toughness. The Izod impact strength provides a good measureof the toughness of a material. A good simulation material will havetoughness in a range that is close to that of the simulated material.For the discussion herein, the impact strength is measured by thenotched Izod test, according to ASTM Test D-256A.

[0047] In general, useful articles are not really used to the point ofbreaking. For example, if a squeeze bottle is made of a material thatbreaks during normal use it will have little value. And in general,useful articles are not often used such that they are stressed pasttheir yield capabilities. For example, if bridges were designed towithstand normal loads, such as a car, which induced stresses in supportmembers exceeding the yield point, the bridge would increase in sag forevery car that passed over it. Exceptions may be found for someapplications such as, for example, living hinges. In these cases, oftenthe first use of the article induces a stress that exceeds the yield ofthe material, but subsequent stresses remain for the most part withinthe elastic range of the material. For the purposes of the instantinvention, material property values relating to the break of a material,for a material having a yield point, are of little interest in terms ofsimulating the look and feel of a simulated material.

[0048] The tensile stress usually quoted is the maximum tensile stress,which is either the stress at yield or the stress at break. If thematerial breaks before it yields, the tensile stress at break of thesimulation material should be compared to the tensile yield stress ofthe simulated material. If the simulation material exhibits a yieldpoint, the tensile yield stress of the simulation material should becompared to the tensile yield stress of the simulated material. In thecase of low to medium density polyethylene, the tensile stress at yieldis 13-28 N/mm2. Simulation materials which have a break tensile stress(before yield) or a tensile yield stress of less than 13 N/mm2, areprobably not suitable as a simulation material for low to medium densitypolyethylene.

[0049] The tensile modulus (and/or the flexural modulus) is probably themost important physical property-with respect to the feel of a material.People can generally feel the stiffness of a material and can tell ifthe material is not stiff enough or if the material is too stiff. Thisis because the modulus is a material property that is determined in theworking range of a material (i.e. prior to plastic deformation of thematerial) and is a material property that can be felt or measured atrelatively low stress levels. In general, a suitable simulation materialhas a tensile modulus which is within the range of moduli of thesimulated material. Low density to medium density polyethylene has atensile modulus range of from approximately 260 to 520 N/mm2. It hasbeen found that simulation compositions resulting in parts having atensile modulus in the range of about 180 to about 850 N/mm² aresuitable simulation materials. Parts having a modulus below that rangeare generally too soft and pliable to have any utility as a polyethylenesimulation. Conversely, parts having a modulus above that range are toostiff. Preferably, the compositions result in parts having a tensilemodulus in the range of about 220 to about 650 N/mm².

[0050] In the case of the most preferred simulation material forpolyethylene, it has been discovered that variations in the exposureduring the solid imaging process lead to significant variations in thetensile modulus. For example, by doubling the exposure of this materialduring the solid imaging process the tensile modulus doubled but theelongation at yield remained unchanged. This is extremely advantageousfor a simulation material since the modulus can be varied over a rangethat very closely matches the modulus range of the simulated material.Such a simulation material is therefore adaptable to simulate variousmolecular weights and grades of polyethylene, for example.

[0051] The elongation properties of a simulation material are alsoimportant. If the simulation material has a tensile elongation at breakthat is lower than the minimum tensile elongation at yield of thesimulated material, it is regarded as not suitable. If the material hasa yield point, the tensile elongation at yield of the simulationmaterial is compared with the tensile elongation at yield of thesimulated material. If the material does not have a yield point, thetensile elongation at break of the simulation material is compared withthe tensile elongation at yield of the simulated material. Suitablesimulation materials have tensile elongation at yield or tensileelongation at break (before yield) values that equal or exceed thetensile elongation at yield values of the simulation material. Low tomedium density polyethylene has a tensile elongation at yield range of6-8%. Therefore a suitable simulation material for polyethylene willhave a tensile elongation at break (before yield) or a tensileelongation at yield of greater than 6%.

[0052] The impact resistance of a simulation material relative to theimpact resistance of a simulated material is also of some importance.For example, it is not unusual for someone handling an object to knockthe object against the corner of a table. From such treatment a feel ofthe materials toughness and sound qualities (deadening, ringing, etc)can be garnered. For the purposes of this patent, a suitable simulationmaterial will have an Izod impact strength that is nearly as strong asthe Izod impact strength of the simulation material. Low to mediumdensity polyethylene has a notched Izod Impact Strength of 53.4 J/m toNo Break. Therefore a suitable simulation material for low to mediumdensity polyethylene has Notched Izod Impact Strength of at least 50J/m.

[0053] The appearance of a simulation material is also an importantconsideration. Low to medium density polyethylene has a cloudyappearance. Therefore a suitable simulation material for low to mediumdensity polyethylene should also have a cloudy appearance and as much aspossible for UV cured materials, minimum color.

[0054] The compositions of the invention generally comprise anepoxide-containing material, a free-radical polymerizable aromaticand/or cycloaliphatic acrylic material, a reactive hydroxyl-containingmaterial, a cationic photoinitiator and a free-radical photoinitiator.

[0055] The epoxide-containing materials that are used in thecompositions, according to this invention, are compounds that possess onaverage at least one 1,2-epoxide group in the molecule. By “epoxide” ismeant the three-membered ring

[0056] The epoxide-containing materials, also referred to as epoxymaterials, are cationically curable, by which is meant thatpolymerization and/or crosslinking and other reaction of the epoxy groupis initiated by cations. The materials can be monomers, oligomers orpolymers and are sometimes referred to as “resins.” Such materials mayhave an aliphatic, aromatic, cycloaliphatic, araliphatic or heterocyclicstructure; they comprise epoxide groups as side groups, or those groupsform part of an alicyclic or heterocyclic ring system. Epoxy resins ofthose types are generally known and are commercially available.

[0057] The epoxide-containing material (a) may be a solid or a liquidwhich is soluble or dispersible in the remaining components. It ispreferred that the epoxide-containing material comprise at least oneliquid component such that the combination of materials is a liquid. Inthe preferred case, the epoxide-containing material can be a singleliquid epoxy material, a combination of liquid epoxy materials, or acombination of liquid epoxy material(s) and solid epoxy material(s)which is soluble in the liquid.

[0058] Examples of suitable epoxy materials include polyglycidyl andpoly(-methylglycidyl) esters of polycarboxylic acids. The polycarboxylicacid can be aliphatic, such as, for example, glutaric acid, adipic acidand the like; cycloaliphatic, such as, for example, tetrahydrophthalicacid; or aromatic, such as, for example, phthalic acid, isophthalicacid, trimellitic acid, or pyromellitic acid. It is likewise possible touse carboxy-terminated adducts, for example, of trimellitic acid andpolyols, such as, for example, glycerol or2,2-bis(4-hydroxycylohexyl)propane.

[0059] Suitable epoxy materials also include polyglycidyl orpoly(-methylglycidyl) ethers obtainable by the reaction of a compoundhaving at least two free alcoholic hydroxy groups and/or phenolichydroxy groups and a suitably substituted epichlorohydrin. The alcoholscan be acyclic alcohols, such as, for example, ethylene glycol,diethylene glycol, and higher poly(oxyethylene) glycols; cycloaliphatic,such as, for example, 1,3- or 1,4-dihydroxycyclohexane,bis(4-hyroxycylohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane, or1,1-bis(hydroxymethyl)cyclohex-3-ene; or contain aromatic nuclei, suchas N,N-bis(2-hydroxyethyl)aniline orp,p′-bis(2-hydroxyethylamino)diphenylmethane.

[0060] The epoxy compounds may also be derived from mono nuclearphenols, such as, for example, from resorcinol or hydroquinone, or theyare based on polynuclear phenols, such as, for example,bis(4-hydroxyphenyl)methane (bisphenol F),2,2-bis(4-hydroxyphenyl)propane (bisphenol A), or on condensationproducts, obtained under acidic conditions, of phenols or cresols withformaldehyde, such-as phenol novolacs and cresol novolacs.

[0061] Suitable epoxy materials also include poly(N-glycidyl) compoundsare, for example, obtainable by dehydrochlorination of the reactionproducts of epichlorohydrin with amines that comprise at least two aminehydrogen atoms, such as, for example, n-butylamine, aniline, toluidine,m-xylylene diamine, bis(4-aminophenyl)methane orbis(4-methylaminophenyl)methane. The poly(N-glycidyl) compounds alsoinclude, however, N,N′-diglycidyl derivatives of cycloalkyleneureas,such as ethyleneurea or 1,3-proyleneurea, and N,N′-diglycidylderivatives of hydantoins, such as of 5,5-dimethylhydantoin.

[0062] Examples of suitable epoxy materials include poly(S-glycidyl)compounds which are di-S-glycidyl derivatives which are derived fromdithiols, such as, for example, ethane-1,2-dithiol orbis(4-mercaptomethylphenyl) ether.

[0063] Examples of suitable epoxy materials include epoxy compounds inwhich the epoxy groups form part of an alicyclic or heterocyclic ringsystem are bis(2,3-epoxycyclopentyl)ether, 2,3-epoxy cyclopentylglycidyl ether, 1,2-bis(2,3-epoxycyclopentyloxy)ethane,bis(4-hydroxycyclohexyl)methane diglycidyl ether,2,2-bis(4-hyroxycylohexyl)propane diglycidyl ether,3,4-epoxycyclo-hexylmethyl-3,4-epoxycyclohexane,3,4-epoxy-6-methyl-cyclohexyl-methyl-3,4-epoxy-6-methylcyclohexanecaboxylate,di-(3,4-epoxycyclohexylmethyl)hexanedioate,di-(3,4-epoxy-6-methyl-cyclohexylmethyl)hexanedioate,ethylenebis(3,4-epoxycyclohexanecarboxylate),ethanedioldi-(3,4-epoxycyclohexylmethyl)ether, vinylcyclohexene dioxide,dicyclopentadiene diepoxide, or2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-1,3-dioxane.

[0064] It is, however, also possible to use epoxy resins in which the1,2-epoxy groups are bonded to different hetero atoms or functionalgroups. Those compounds include, for example, the N,N,O-triglycidylderivative of 4-aminophenol, the glycidyl ether glycidyl ester ofsalicylic acid,N-glycidyl-N′-(2-glycidyloxypropyl)-5,5-dimethylhydantoin, or2-glycidyloxy-1,3-bis(5,5-dimethyl-1-glycidylhydantoin-3-yl)propane.

[0065] In addition, liquid pre-reacted adducts of such epoxy resins withhardeners are suitable for epoxy resins.

[0066] It is of course also possible to use mixtures of epoxy materialsin the compositions according to the invention.

[0067] Preferred epoxy materials are cycloaliphatic diepoxides.Especially preferred are bis(4-hydroxycyclohexyl)methane diglycidylether, 2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether,3,4-epoxycyclohexylmethyl-3,4 epoxycyclohexanecarboxylate,3,4-epoxy-6-methyl-cyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate,di-(3,4-epoxycyclohexylmethyl)hexanedioate,di-(3,4-epoxy-6-methyl-cyclohexylmethyl)hexanedioate,ethylenebis(3,4-epoxycyclohexanecarboxylate),ethanediol-di-(3,4-epoxycyclohexylmethyl) ether, and2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-1,3-dioxane.

[0068] The epoxy materials can have molecular weights which vary over awide range. In general, the epoxy equivalent weight, i.e., the numberaverage molecular weight divided by the number of,reactive epoxy groups,is preferably in the range of about 60 to about 1000.

[0069] The free-radical polymerizable acrylic materials that are used inthe composition, according to this invention, are aromatic and/orcycloaliphatic compounds that have, on average, at least one acrylicgroup which can be either the free acid or an ester. By “acrylic” ismeant the group —CH═CR¹-CO₂R², where R¹ can be hydrogen or methyl and R²can be hydrogen or alkyl. By “(meth)acrylate” is meant a acrylate,methacrylate or combinations thereof. The acrylic materials undergopolymerization and/or crosslinking reactions initiated by free radicals.The acrylic materials can be monomers, oligomers or polymers. It ispreferred that the acrylic material be a monomer or oligomer.

[0070] Suitable as the acrylic component are, for example, thediacrylates of cycloaliphatic or aromatic diols, such as1,4-dihydroxymethylcyclohexane, 2,2-bis(4-hydroxycycyclohexyl)propane,bis(4hydroxycyclohexyl)methane, hydroquinone, 4,4-dihydroxybiphenyl,bisphenol A, bisphenol F, bisphenol S, ethoxylated or propoxylatedbisphenol A, ethoxylated or propoxylated bisphenol F, or ethoxylated orpropoxylated bisphenol S. Such acrylates are known and some of them arecommercially available.

[0071] Preferred are compositions comprising as the acrylic component acompound of formula I, II, III or IV

[0072] wherein

[0073] Y is a direct bond, C1-C6 alkylene, —S—, —O—, —SO—, —SO2—, or—CO—, R10 is a C1-C8 alkyl group, a phenyl group that is unsubstitutedor substituted by one or more C1-C4 alkyl groups, hydroxy groups orhalogen atoms, or a radical of the formula CH₂-R11, wherein R11 is aC1-C8 alkyl group or a phenyl group, and A is a radical of the formula

[0074] or comprising as the acrylic component a compound of any one offormulae Va to Vd,

[0075] and the corresponding isomers,

[0076] If a substituent is C1-C4 alkyl or C1-C8 alkyl, it may bestraight-chained or branched. A C1-C4 alkyl may be, for example, methyl,ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl,and a C1-C8 alkyl may additionally be, for example, the various pentyl,hexyl, heptyl, or octyl isomers.

[0077] If a substituent is halogen, it is fluorine, chlorine, bromine,or iodine, but especially chlorine or bromine.

[0078] If a substituent is C1-C6 alkylene it is, for example, methylene,ethylene, propylene (methylethylene), trimethylene, 1,1-propanediyl,2,2-propanediyl, tetramethylene, ethylmethylene, 1,1-butanediyl,2,2-butanediyl, pentamethylene or hexamethylene. The alkylene radicalsmay also be substituted by halogen atoms. Examples of halogenatedalkylene radicals are —C(CC1₃)₂— and —C(CF₃)₂—.

[0079] Especially preferred in the compositions are compounds of theformula V, VI or VII wherein Y is —CH₂— or —(CH₃)₂—. Also especiallypreferred are compounds of formula VII wherein R10 is n-butyl, phenyl,n-butoxymethyl, or phenoxymethyl.

[0080] Suitable as aromatic tri(meth-)acrylates are, for example, thereaction products of triglycidyl ethers of trihydric phenols, and phenolor cresol novolacs having three hydroxy groups with (meth)-acrylic acid.

[0081] Compositions wherein the acrylic component is an acrylate ofbisphenol A diepoxide such as Ebecryl® 3700 from UCB ChemicalCorporation, Smyrna, Georgia or an acrylate of 1,4-cyclohexanedimethanolare especially preferred for compositions used in this invention.

[0082] In addition to the aromatic or cycloaliphatic acrylic material(b), other acrylic materials can be present. Liquid poly(meth-)acrylateshaving functionality of greater than 2 which may, where appropriate, beused in the compositions according to the invention. These can be, forexample, tri-, tetra-, or penta-functional monomeric or oligomericaliphatic, (meth)acrylates.

[0083] Suitable as aliphatic polyfunctional (meth)acrylates are, forexample, the triacrylates and trimethacrylates of hexane-2,4,6-triol,glycerol, or 1,1,1-trimethylolpropane, ethoxylated or propoxylatedglycerol, or 1,1,1-trimethylolpropane and the hydroxy group-containingtri(meth)acrylates which are obtained by the reaction of triepoxycompounds, such as, for example, the triglycidyl ethers of the mentionedtriols, with (meth)acrylic acid. It is also possible to use, forexample, pentaerythritol tetra-acrylate, bistrimethylolpropanetetra-acrylate, pentaerythritol monohydroxytri(meth)acrylate, ordipentaerythritol monohydroxypenta(meth)acrylate.

[0084] It is also possible to use hexafunctional urethane(meth)acrylates. Those urethane (meth)acrylates are known to the personskilled in the art and can be prepared in known manner, for example byreacting a hydroxy-terminated polyurethane with acrylic acid ormethacrylic acid, or by reacting an isocyanate-terminated prepolymerwith hydroxyalkyl (meth)acrylates to follow the urethane (meth)acrylate.Also useful are (meth)acrylates such as tris(2-hydroxyethyl)isocyanuratetriacrylate.

[0085] The reactive hydroxyl-containing material which is used in thepresent invention may be any organic material having hydroxylfunctionality of at least 1, and preferably at least 2. The material maybe a liquid or a solid that is soluble or dispersible in the remainingcomponents. The material should be substantially free of any groupswhich inhibit the curing reactions, or which are thermally orphotolytically unstable.

[0086] Preferably the organic material contains two or more primary orsecondary aliphatic hydroxyl groups, by which is meant that the hydroxylgroup is bonded directly to a non-aromatic carbon atom. The hydroxylgroup may be internal in the molecule or terminal. Monomers, oligomersor polymers can be used. The hydroxyl equivalent weight, i.e., thenumber average molecular weight divided by the number of hydroxylgroups, is preferably in the range of about 31 to 5000.

[0087] Representative examples of suitable organic materials having ahydroxyl functionality of 1 include alkanols, monoalkyl ethers ofpolyoxyalkyleneglycols, monoalkyl ethers of alkylene-glycols, andothers.

[0088] Representative examples of useful monomeric polyhydroxy organicmaterials include alkylene and arylalkylene glycols and polyols, such as1,2,4-butanetriol, 1,2,6-hexanetriol, 1,2,3-heptanetriol,2,6-dimethyl-1,2,6-hexanetriol,(2R,3R)-(-)-2-benzyloxy-1,3,4-butanetriol, 1,2,3-hexanetriol,1,2,3-butanetriol, 3-methyl-1,3,5-pentanetriol, 1,2,3-cyclohexanetriol,1,3,5-cyclohexanetriol, 3,7,11,15-tetramethyl-1,2,3-hexadecanetriol,2-hydroxymethyl-tetrahydro-pyran-3,4,5-triol,2,2,4,4-tetramethyl-1,3-cyclobutanediol, 1,3-cyclopentanediol,trans-1,2-cyclooctanediol, 1,16-hexadecanediol,3,6-dithia-1,8-octanediol, 2-butyne-1,4-diol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol,1,8-octanediol, 1,9-nonanediol, l-phenyl-1,2-ethanediol,1,2-cyclohexanediol, 1,5-decalindiol, 2,5-dimethyl-3-hexyne-2,5-diol,2,7-dimethyl-3,5-octadiyne-2-7-diol, 2,3-butanediol,1,4-cyclohexanedimethanol.

[0089] Representative examples of useful oligomeric and polymerichydroxyl-containing materials include polyoxyethylene andpolyoxypropylene glycols and triols of molecular weights from about 200to about 10,000; polytetramethylene glycols of varying molecular weight;copolymers containing pendant hydroxy groups formed by hydrolysis orpartial hydrolysis of vinyl acetate copolymers, polyvinylacetal resinscontaining pendant hydroxyl groups; hydroxy-terminated polyesters andhydroxy-terminated polylactones; hydroxy-functionalized polyalkadienes,such as polybutadiene; and hydroxy-terminated polyethers.

[0090] Preferred hydroxyl-containing monomers are1,4-cyclohexanedimethanol and aliphatic and cycloaliphatic mono-hydroxyalkanols.

[0091] Preferred hydroxyl-containing oligomers and polymers includehydroxyl and hydroxyl/epoxy functionalized polybutadiene,polycaprolactone diols and triols, ethylene/butylene polyols, andcombinations thereof. Preferred examples of polyether polyols arepolypropylene glycols of various molecular weights and glycerolpropoxylate-B-ethoxylate triol. Especially preferred are linear andbranched polytetrahydrofuran polyether polyols available in variousmolecular weights, such as for example 250, 650, 1000, 2000, and 2900MW.

[0092] In the compositions according to the invention, any type ofphotoinitiator that, upon exposure to actinic radiation, forms cationsthat initiate the reactions of the epoxy material(s) can be used. Thereare a large number of known and technically proven cationicphotoinitiators for epoxy resins that are suitable. They include, forexample, onium salts with anions of weak nucleophilicity. Examples arehalonium salts, iodosyl salts or sulfonium salts, such as are describedin published European patent application EP 153904, sulfoxonium salts,such as described, for example, in published European patentapplications EP 35969, 44274, 54509, and 164314, or diazonium salts,such as described, for example, in U.S. Pat. Nos. 3,708,296 and5,002,856. Other cationic photoinitiators are metallocene salts, such asdescribed, for example, in published European applications EP 94914 and94915.

[0093] A survey of other current onium salt initiators and/ormetallocene salts can be found in “UV-Curing, Science and Technology”,(Editor S. P. Pappas, Technology Marketing Corp., 642 Westover Road,Stanford, Conn., U.S.A.) or “Chemistry & Technology of UV & EBFormulation for Coatings, Inks & Paints”, Vol. 3 (edited by P. K. T.Oldring).

[0094] Preferred cationic photoinitiators are compounds of formula VI,VII, or VIII below,

[R₁—I—R₂]⁺[Q_(m)]³¹   (VI)

[0095]

[0096] wherein:

[0097] R₁, R₂, R₃, R₄, R₅, R₆, and R₇ are each independently of theothers C6-C18 aryl that is unsubstituted or substituted by suitableradicals,

[0098] L is boron, phosphorus, arsenic, or antimony,

[0099] Q is a halogen atom or some of the radicals Q in an anion LQ_(m)⁻may also be hydroxy groups, and

[0100] m is an integer that corresponds to the valence of L plus 1.

[0101] Examples of C6-C18 aryl are phenyl, naphthyl, anthryl, andphenanthryl. Any substituents present for suitable radicals are alkyl,preferably C1-C6 alkyl, such as methyl, ethyl, n-propyl, isopropyl,n-butyl, sec-butyl, isobutyl, tert-butyl, or the various pentyl or hexylisomers, alkoxy, preferably C1-C6 alkoxy such as methoxy, ethoxy,propoxy, butoxy, pentyloxy, or hexyloxy, alkylthio, preferably C1-C6alkylthio, such as methylthio, ethylthio, propylthio, butylthio,pentylthio, or hexylthio, halogen, such as fluorine, chlorine, bromine,or iodine, amino groups, cyano groups, nitro groups, or arylthio, suchas phenylthio.

[0102] Examples of preferred halogen atoms Q are chlorine and especiallyfluorine. Preferred anions LQ_(m) ⁻are BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, andSbF₅(OH)⁻.

[0103] Especially preferred are compositions comprising as the cationicphotoinitiator a compound of formula VIII wherein R5, R6 and R7 arearyl, aryl being especially phenyl or biphenyl, or mixtures of those twocompounds.

[0104] Also preferred are compositions comprising as the cationicphotoinitiator component compounds of formula

[R₈(Fe^(II)R₉)_(c)]_(d) ^(+c)[x]^(−d),  (IX)

[0105] wherein,

[0106] c is 1 or 2,

[0107] d is 1,2,3,4 or 5,

[0108] X is a non-nucleophilic anion, especially -PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻,CF₃SO₃ ⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO₃ ⁻, n-C₄F₉SO₃ ⁻,

[0109] n-C₆F₁₃SO₃ ⁻, or n-C₈F₁₇SO₃ ⁻, R8 is a pi-arene, and R9 is ananion of a pi-arene, especially a cyclopentadienyl anion.

[0110] Examples of pi-arenes as R8 and anions of pi-arenes as R9 are tobe found in published European patent application EP 94915.

[0111] Examples of preferred pi-arenes as R8 are toluene, xylene,ethylbenzene, cumene, methoxybenzene, methylnaphthalene, pyrene,perylene, stilbene, diphenylene oxide and diphenylene sulfide.Especially preferred are cumene, methylnaphthalene, or stilbene.

[0112] Examples of non-nucleophilic anions X³¹ are FSO₃ ⁻, anions oforganic sulfonic acids, of carboxylic acids, or anions LQ_(m) ⁻, asalready defined above.

[0113] Preferred anions are derived from partially fluoro- orperfluoro-aliphatic or partially fluoro- or perfluoro-aromaticcarboxylic acids, or especially from partially fluoro- orper-fluoro-aliphatic or partially fluoro or perfluoro-aromatic organicsulfonic acids, or they are preferably anions LQ_(m) ⁻.

[0114] Examples of anions X are BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, SbF₅(OH)⁻,CF₃SO₃ ⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO₃ ⁻, n-C₄F₉SO₃ ⁻, n-C₆F₁₃SO₃ ⁻, n-C₈F₁₇SO₃⁻, C₆F₅SO₃ ⁻, phosphorus tungstate, or silicon tungstate. Preferred arePF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, CF₃SO⁻, C₂F₅SO₃ ⁻, n-C₃F₇SO₃ ⁻, n-C₄F₉SO₃ ⁻,n-C₆F₁₃SO₃ ⁻, and n-C₈F₁₇SO₃ ⁻.

[0115] The metallocene salts can also be used in combination withoxidizing agents. Such combinations are described in published Europeanpatent application EP 126712.

[0116] In order to increase the light efficiency, or to sensitize thecationic photoinitiator to specific wavelengths, such as for examplespecific laser wavelengths or a specific series of laser wavelengths, itis also possible, depending on the type of initiator, to usesensitizers. Examples are polycyclic aromatic hydrocarbons or aromaticketo compounds. Specific examples of preferred sensitizers are mentionedin published European patent application EP 153904. Other preferredsensitizers are benzo[g,h,i]perylene, 1,8-diphenyl-1,3,5,7-octatetraene,and 1,6-diphenyl-1,3,5-hexatriene as described in U.S. Pat. No.5,667,937. It will be recognized that an additional factor in the choiceof sensitizer is the nature and primary wavelength of the source ofactinic radiation.

[0117] In the compositions according to the invention, any type ofphotoinitiator that forms free radicals when the appropriate irradiationtakes place can be used. Typical compounds of known photoinitiators arebenzoins, such as benzoin, benzoin ethers, such as benzoin methyl ether,benzoin ethyl ether, and benzoin isopropyl ether, benzoin phenyl ether,and benzoin acetate, acetophenones, such as acetophenone,2,2-dimethoxyacetophenone, 4-(phenylthio)acetophenone, and1,1-dichloroacetophenone, benzil, benzil ketals, such as benzil dimethylketal, and benzil diethyl ketal, anthraquinones, such as2-methylanthraquinone, 2-methylanthraquinone, 2-tert-butylanthraquinone,1-chloroanthraquinone, and 2-amylanthraquinone, also triphenylphosphine,benzoylphosphine oxides, such as, for example,2,4,6-trimethylbenzoyldiphenylphosphine oxide (Lucirin® TPO),benzophenones, such as benzophenone, and4,4′-bis(N,N′-dimethylamino)benzophenone, thioxanthones and xanthones,acridine derivatives, phenazene derivatives, quinoxaline derivatives or1-phenyl-1,2-propanedione-2-O-benzoyloxime, 1-aminophenyl ketones or1-hydroxyphenyl ketones, such as 1-hydroxycyclohexyl phenyl ketone,phenyl (1-hydroxyisopropyl)ketone and4-isopropylphenyl(1-hydroxyisopropyl)ketone, or triazine compounds, forexample, 4″′methyl thiophenyl-1-di(trichloromethyl)-3,5 S-triazine,S-triazine-2-(stylbene)-4,6-bis-trichloromethyl, and paramethoxy stiryltriazine, all of which are known compounds.

[0118] Especially suitable free-radical photoinitiators, which arenormally used in combination with a He/Cd laser, operating at forexample 325 nm, an Argon-ion laser, operating at for example 351 nm, or351 and 364 nm, or 333, 351, and 364 nm, or a frequency tripled YAGsolid state laser, having an output of 351 or 355 nm, as the radiationsource, are acetophenones, such as 2,2-dialkoxybenzophenones and1-hydroxyphenyl ketones, for example 1-hydroxycyclohexyl phenyl ketone,2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl)-2-methyl-1-propanone, or2-hydroxyisopropyl phenyl ketone (also called2-hydroxy-2,2-dimethylacetophenone), but especially 1-hydroxycyclohexylphenyl ketone. Another class of free-radical photoinitiators comprisesthe benzil ketals, such as, for example, benzil dimethyl ketal.Especially an alpha-hydroxyphenyl ketone, benzil dimethyl ketal, or2,4,6-trimethylbenzoyldiphenylphosphine oxide is used as photoinitiator.

[0119] Another class of suitable free radical photoinitiators comprisesthe ionic dye-counter ion compounds, which are capable of absorbingactinic rays and producing free radicals, which can initiate thepolymerization of the acrylates. The compositions according to theinvention that comprise ionic dye-counter ion compounds can thus becured in a more variable manner using visible light in an adjustablewavelength range of 400 to 700 nanometers. Ionic dye-counter ioncompounds and their mode of action are known, for example from publishedEuropean patent application EP 223587 and U.S. Pat. Nos. 4,751,102,4,772,530 and 4,772,541. There may be mentioned as examples of suitableionic dye-counter ion compounds the anionic dye-iodonium ion complexes,the anionic dye-pyryllium ion complexes and, especially, the cationicdye-borate anion compounds of the following formula

[0120] wherein D+ is a cationic dye and R₁₂, R₁₃, R₁₄, and R₁₅ are eachindependently of the others arkyl, aryl, alkaryl, allyl, aralkyl,alkenyl, alkynyl, an alicyclic or saturated or unsaturated heterocyclicgroup. Preferred definitions for the radicals R₁₂ to R₁₅ can be found,for example, in published European patent application EP 223587.

[0121] Especially preferred is the free-radical photoinitiator1-hydroxyphenyl ketone, which produces parts having the least amount ofyellowing after final cure and provides articles which most closelysimulate polyethylene.

[0122] Other additives which are known to be useful in solid imagingcompositions may also be present in the composition of the invention.Stabilizers are often added to the compositions in order to prevent aviscosity build-up during usage in the solid imaging process. Thepreferred stabilizers are described in U.S. Pat. No. 5,665,792. Suchstabilizers are usually hydrocarbon carboxylic acid salts of group IAand IIA metals. Most preferred examples of these salts are sodiumbicarbonate, potassium bicarbonate, and rubidium carbonate. Rubidiumcarbonate is preferred for formulations of this invention withrecommended amounts varying between 0.0015 to 0.005% by weight ofcomposition. Alternative stabilizers are polyvinylpyrrolidones andpolyacrylonitriles. Other possible additives include dyes, pigments,fillers, wetting agents, photosensitizers for the free-radicalphotoinitiator, leveling agents, surfactants and the like.

[0123] The compositions of the invention generally comprise from about20% by weight to less than about 40% by weight of the epoxide-containingmaterial, based on the total weight of the composition.

[0124] It is sometimes beneficial to describe the compositions in termsof equivalents or milliequivalents of epoxy material per 100 grams oftotal composition. The epoxy equivalent weight can be derived bydividing the molecular weight of a molecule by the number of epoxygroups contained within the molecule. The total epoxy equivalent wieghtof a composition is determined by first calculating the epoxy content ofeach component, i.e., epoxide-containing material, epoxy-acrylate, etc.The individual component epoxy equivalent weihgts are weight averagedfor the entire composition. It is preferred that the compositionscomprise from about 250 to about 350 milliequivalents of epoxy per 100grams of composition.

[0125] The compositions of the invention preferably comprise from about5% to about 45% by weight of free-radical polymerizable acrylicmaterial, based on the total weight of the composition. It is mostpreferred that the acrylic be an aromatic and/or cycloaliphaticdiacrylate or di-methacrylate.

[0126] The compositions of the invention preferably comprise from about5% to about 50% by weight of reactive hydroxy-containing material, basedon the total weight of the composition, more preferably from about 20%to about 30% by weight.

[0127] It is sometimes beneficial to describe the compositions in termsof equivalents or milliequivalents of hydroxyl-containing material per100 grams of total composition. The hydroxyl equivalent weight can bederived by dividing the molecular weight of a molecule by the number ofhydroxyl groups contained within the molecule. The total number ofequivalent of hydroxyl in a composition is determined by firstcalculating the hydroxyl content of each component, i.e.,epoxide-containing material, epoxy-acrylate, polyol, initiator, etc. Theindividual component hydroxyl equivalent weights are weight averaged forthe entire composition. All hydroxyls are assumed to be reactive,regardless of steric hindrance. It is preferred that the compositioncomprise from about 140 to about 180 milliequivalents of reactivehydroxy-containing material per 100 grams of composition. It is mostpreferred that the ratio of epoxy equivalents to hydroxyl equivalents bein the range of from about 1.5 to about than 2.5; most preferably 1.9 to2.4.

[0128] The compositions of the invention preferably comprise from about0.2 to about 10% by weight of cationic photoinitiator, based on thetotal weight of the composition.

[0129] The compositions of the invention preferably comprise from about0.01 to about 10% by weight of free-radical photoinitiator, based on thetotal weight of the composition.

[0130] The compositions of the invention can be prepared according toconventional procedures. In general, the components are combined bymixing in any suitable mixing apparatus. In some cases, some componentscan be premixed before adding to the total composition. In some cases,the mixing is carried out in the absence of light. In some cases, themixing is carried out with some heating, generally at temperatures thatrange from about 30° C. to about 60° C.

[0131] The process for producing three-dimensional articles from thecompositions of the invention, as discussed above, generally involvesexposure of successive thin layers of the liquid composition to actinicradiation. A thin layer of the photosensitive composition of theinvention is coated onto a surface. This is most conveniently done ifthe composition is a liquid. However, a solid composition can be meltedto form a layer. The thin layer is then exposed imagewise to actinicradiation. The radiation must provide sufficient exposure to causesubstantial curing of the photosensitive composition in the exposedareas. By “substantial curing” it is meant that the photosensitivecomposition has reacted to an extent such that the exposed areas arephysically differentiable from the unexposed areas. For liquid, gel orsemi-solid photosensitive compositions, the cured areas will havehardened or solidified to a non-fluid form. For solid photosensitivecompositions, the exposed areas will have a higher melting point thanthe non-exposed areas. Preferably, the exposure is such that portions ofeach successive layer are adhered to a portion of a previously exposedlayer or support region, or to portions of a platform surface. Thisfirst exposure step forms a first imaged cross-section. An additional(second) thin layer of photosensitive composition is then coated ontothe first imaged cross-section and imagewise exposed to actinicradiation to form an additional (second) imaged cross-section. Thesesteps are repeated with the “nth” thin layer of photosensitivecomposition being coated onto the “(n-1)th” imaged cross-section andexposing to actinic radiation. The repetitions are carried out asufficient number of times to build up the entire three-dimensionalarticle.

[0132] The radiation is preferably in the range of 280-650 nm. Anyconvenient source of actinic radiation can be used, but lasers areparticularly suitable. Useful lasers include HeCd, argon, nitrogen,metal vapor, and NdYAG lasers. The exposure energy is preferably in therange of about 10-75 mJ/cm². Suitable methods and apparatus for carryingout the exposure and production of three-dimensional articles have beendescribed in, for example, U.S. Pat. Nos. 4,987,044, 5,014,207, and5,474,719, which teaches the use of pseudoplastic, plastic flow,thixotropic, gel, semi-solid and solid photopolymer materials in thesolid imaging process.

[0133] In general, the three-dimensional article formed by exposure toactinic radiation, as discussed above, is not fully cured, by which ismeant that not all of the reactive material in the composition hasreacted. Therefore, there is often an additional step of more fullycuring the article. This can be accomplished by further irradiating withactinic radiation, heating, or both. Exposure to actinic radiation canbe accomplished with any convenient radiation source, generally a UVlight, for a time ranging from about 10 to over 60 minutes. Heating isgenerally carried out at a temperature in the range of about 75-150° C.,for a time ranging from about 10 to over 60 minutes.

EXAMPLES

[0134] The components 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (epoxy); polytetrahydrofuran linear chain (1000 mw)(polyTHF); l,4-cyclohexanedimethanol (CHDM); 1-hydroxycyclohexyl phenylketone (free-radical initiator, FRI-1);2-hydroxy-1-(4-(2-hydroxyethoxy)phenyl}-2-methyl-1-propanone(free-radical initiator, FRI-2); trimethylolpropane triacrylate (TMPTA);and 2,2-dimethoxy-2-phenylacetophenone (free-radical initiator, FRI-3)are available from Aldrich Chemical Company Inc (Milwaukee, Wis.). Theacrylate ester of bisphenol-A epoxy (Ebecryl) is available from UCBChemicals Corp. (Smyrna, Ga.) as Ebecryl® 3700. The1,4-cyclohexanedimethanol diacrylate esters (CHDMDA) are sold bySartomer Company (Exton, Pa.). The mixed triarylsulfoniumhexafluoroantimonate salts in 50% by weight propylene carbonate(cationic initiator, CatI) is sold by Union Carbide Chemicals andPlastics Company Inc. (Danbury, Conn.). Somos® 2100, 3110, and 7100 arecommercial products sold by E. I. du Pont de Nemours and Company(Wilmington, Del.).

[0135] The individual components were weighed out, combined, then heatedto 50° C. and mixed for several hours until all the ingredients werecompletely dissolved.

[0136] For all formulations, the exposure-working curve of the formulawas determined using methods well known in the art. The working curve isa measure of the photospeed of the particular material. It representsthe relationship between the thickness of a floating layer, scanned onthe surface of the photopolymer in a vat or petri-dish, produced as afunction of the exposure given. Parts were fabricated by forming aseries of 6 mil (0.15 mm) coated layers, and giving enough imagewiseexposure to each layer to create a cure that would correspond to a 10mil (0.254 mm) working curve thickness.

[0137] The exposures used to create the tensile and Izod impact testparts are given in Table 2. Unless otherwise indicated, all parts werefabricated using an Argon Ion laser operating with an output of 351 nm.

[0138] After the parts were formed, they were cleaned in a solvent,allowed to dry and then fully cured. All parts, except the Somos® 2100part, were given a UV postcure for 60 minutes in a Post CuringApparatus, manufactured by 3D Systems, Inc. (Valencia, Calif.). TheSomos® 2100 part was postcured in an oven for 30 minutes at 150 degreesC. then UV postcured for 30 minutes.

[0139] All tensile properties were measured according to ASTM TestD-638M. For the Somos® samples, the temperature and humidity werecontrolled as specified. The temperature and humidity of the Exampleparts were not controlled during testing. However, the temperature wasapproximately 20-22° C. and the humidity was approximately 20-30% RH.

[0140] The impact stength of all the samples was measured by theknotched Izod test, according to ASTM Test D-256A.

[0141] The physical test values for polyethylene were obtained fromvarious sources. The values of Tensile Stress at Break, Tensile YieldStress, Tensile Elongation at Break, Notched Izod Impact, and TensileModulus are for polyethylene and ethylene copolymers (low and mediumdensity, linear copolymer) as listed in Modern Plastics Encyclopedia'98, Mid-November 1997 Issue, The McGraw-Hill Companies, Inc., New York,N.Y. The values of Tensile Elongation at Yield for polyethylene wereobtained from the Internet address http://www.chemopetrol.cz/docs/en₁₃catal/ published by Chemopetrol in Czechoslovakia. The value ranges weredetermined by incorporating the information from Chemopetrol's variousLinten and Mosten grades of polyethylene offerings. The range ofelongation at yield for polyethylene may be greater than that providedin Chemopetrol's product literature.

EXAMPLES 1-6

[0142] Compositions according to the invention were prepared having thefollowing components, where “Meq epoxy” indicates the number ofmilliequivalents of epoxy per 100 grams of composition, “Meq hydroxyl”indicates the number of milliequivalents of hydroxyl per 100 grams ofcomposition, and “Ratio” indicates the ratio of milliequivalents ofepoxy to milliequivalents of hydroxyl: Parts by weight Component Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Epoxy 37.00 37.30 39.50 39.65 36.38 37.38Ebecryl 25 25 25 25 25 25 PolyTHF 25 25 25 25 28.5 27.5 CHDM FRI-1 4 — —— 2.8 2.8 FRI-2 — 3.3 — — — — FRI-3 — — 1.2 1.45 — — CHDMA 7 7 7 7 7 7CatI 2 2.4 2.3 1.9 0.32 0.32 Meq epoxy 274 276 293 294 269 277 Meqhydroxyl 165 175 145 145 166 164 Ratio 1.66 1.58 2.01 2.02 1.62 1.69

[0143] The compositions of the invention were exposed and tested asdescribed above. Examples 1-4 were exposed at 351 nm. Examples 5 and 6were exposed at 325 nm. The results are given in Table 1 below.

Comparative Examples C1-C2

[0144] Comparative compositions were prepared having the followingcomponents, where “Meq epoxy” indicates the number of milliequivalentsof epoxy per 100 grams of composition, “Meq hydroxyl” indicates thenumber of milliequivalents of hydroxyl per 100 grams of composition, and“Ratio” indicates the ratio of milliequivalents of epoxy tomilliequivalents of hydroxyl: -- Parts by weight -- Component C1 C2Epoxy 50.20 55.00 Ebecryl 10 — PolyTHF 20 20 CHDM 2.5 6.0 FRI-2 3.0 3.5CatI 2.3 2.5 TMPTA 12 13 Meq epoxy 372 407 Meq hydroxyl 140 155 Ratio2.66 2.64

[0145] The compositions were exposed and tested as described above.Samples of Somos® 2100, 3100 and 3110 were similarly exposed and tested.Comparative Examples 1 and 2 and Somos® 7100 were exposed at 351 nm.Samples of Somos® 3110 were exposed at 325 nm. Samples of Somos® 2100were exposed using a combination of 351 and 364 nm radiation using anArgon ion laser operating with two-line output. The results are given inTable 2 below. TABLE 1 Property PE Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 TYS13-28 27 23 24 28 17 TMod 260-520 275-620 533 625 820 400 TYE % 6-8 21.424.9 26.6 25.4 18 Izod 53.4+ 57 70 N/A 66 N/A Energy N/A 31 50 35 36 47Property Ex. 6 C1 C2 2100 3110 7100 TYS 24 33 45 9 26 62 TMod 745 12821651 90 1282 2068 TYE % 17 4.8 4.7 37 5.7 4.7 Izod N/A N/A 52 90 16 29Energy 48 43 41 13 18 59

[0146] The formulations in Examples 1 and 5 produced hazy parts thatlooked just like low to medium density polyethylene. Lower exposures,such as, for example, 31 mJ/cm², during the solid imaging process,produced parts that were hazier and softer, similar to lower densitypolyethylenes. Higher exposures, such as, for example, 62 mJ/cm², duringthe solid imaging process, produced parts that had less haze and wereharder, similar to medium density polyethylenes. The tensile stress atyield was favorable for Examples 1 and 5 as a simulation material forpolyethylene. The tensile modulus range for Example 1 was a surprisinglygood match for polyethylene. Example 5 will exhibit a similar match forthe tensile modulus range of polyethylene. The tensile break and yieldstrain values of articles made from Examples 1 and 5 were more thanadequate in order to simulate polyethylene articles. The notched Izodimpact strength of Example 1 parts was near the low end, but within therange of polyethylenes. Overall, both were excellent simulationmaterials for polyethylene.

[0147] The formulations from Examples 2-4 and 6 also met most of thecriteria to be good simulation materials for polyethylene. The tensilemodulii were higher than the polyethylene range, but acceptable as asimulation material. Lower exposures during the solid imaging phase offabrication would result in lowering the modulus allowing all to be goodcandidates as simulation materials for polyethylene.

[0148] The formulations in Examples 5 and 6 were variations of Example 1wherein the compositions contained initiator concentrations suitable foruse with a He-Cd laser operating at 325 nm in a solid imaging process.The variations in the Examples involved increases in the3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate content witha concurrent decrease in the polytetrahydrofuran linear chain (1000 MW)content. As can be seen from Table 1, small variations in the3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate content havea remarkable effect on the tensile modulus. This effect is showngraphically in FIG. 2.

[0149] Parts made from the formulations of Comparative Examples C1 andC2 were very stiff. The tensile yield stress and tensile modulus forboth Comparative Examples was much higher than that of polyethylene. Inaddition, the tensile yield elongation was unacceptably low. Therefore,Comparative Examples C1 and C2 are not acceptable as simulationmaterials for polyethylene.

[0150] Somos® 2100 parts generally appeared milky white. On anappearance basis therefore, it is not desirable as a simulation materialfor polyethylene. When comparing the yield and break tensile stressmeasurements for Somos® 2100 against the corresponding values forpolyethylene, it can be seen that Somos® 2100 is too weak. Therefore,Somos® 2100 is regarded as not suitable as a simulation material forpolyethylene.

[0151] Parts made from Somos® 3110 and 7100 were significantly moreclear than those made of polyethylene and the parts made from thesematerials had tensile modulii that were much too high. In addition, thetensile yield elongation was too low to simulate polyethylene.Therefore, these commerical products also are regarded as not suitableas simulation materials for polyethylene.

What is claimed is:
 1. A photosensitive composition comprising: (a)about 20-40% by weight of a epoxide-containing material; (b) about 5-40%by weight of acrylic material selected from aromatic acrylic material,cycloaliphatic acrylic material, or combinations thereof; (c) about5-50% by weight of a reactive hydroxyl-containing material; (d) at leastone cationic photoinitiator; and (e) at least one free-radicalphotoinitiator; with the proviso that upon exposure to actinic radiationan article is produced having the following properties: (i) a tensilebreak before yield stress or a tensile yield stress greater than 13N/mm2; (ii) a tensile modulus in the range of about 180 to about 850N/mm2; (iii) a tensile break elongation before yield or a tensile yieldelongation greater than 6%; and (iv) a notched Izod impact strengthgreater than 50 J/m.
 2. The composition of claim 1 wherein the articleproduced has a cloudy appearance that simulates polyethylene.
 3. Thecomposition of claim 1 wherein the epoxide-containing material isselected from bis(4-hydroxycyclohexyl)methane diglycidyl ether;2,2-bis(4-hydroxycyclohexyl)propane diglycidyl ether;3,4-epoxycyclohexylmethyl-3,4 epoxycyclohexanecarboxylate;3,4-epoxy-6-methyl-cyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate;di-(3,4-epoxycyclohexylmethyl)hexanedioate;di-(3,4-epoxy-6-methyl-cyclohexylmethyl)hexanedioate;ethylenebis(3,4-epoxycyclohexanecarboxylate);ethanediol-di-(3,4-epoxycyclohexylmethyl) ether;2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane-1,3-dioxane; andcombinations thereof.
 4. The composition of claim 1 wherein the acrylicmaterial is selected from 1,4-dihydroxymethylcyclohexane diacrylate;bisphenol A diacrylate; ethoxylated bisphenol A diacrylate; andcombinations thereof.
 5. The composition of claim 1 wherein the reactivehydroxyl-containing material is selected from 1,4-cyclohexanedimethanol;polytetrahydrofuran polyether polyols; and combinations thereof.
 6. Thecomposition of claim 1 wherein the free-radical photoinitiator is a1-hydroxyphenyl ketone.
 7. The composition of claim 1 wherein thearticle produced has a tensile modulus in the range of from about 220 toabout 650 N/mm².
 8. A photosensitive composition comprising: (a) atleast about 20% by weight of a epoxide-containing material; (b) about5-40% by weight of an aromatic or cycloaliphatic acrylic material; (c)at least about 5% by weight of a reactive hydroxyl-containing material;(d) at least one cationic photoinitiator; and (e) at least onefree-radical photoinitiator; wherein the composition comprises fromabout 250 to about 350 milliequivalents of epoxy per 100 grams ofcomposition and from about 140 to about 180 milliequivalents of hydroxylper 100 grams of composition, with the proviso that upon exposure toactinic radiation an article is produced having the followingproperties: (i) a tensile break before yield stress or a tensile yieldstress greater than 13 N/mm2; (ii) a tensile modulus in the range ofabout 180 to about 850 N/mm2; (iii) a tensile break elongation beforeyield or a tensile yield elongation greater than 6%; and (iv) a notchedIzod impact strength greater than 50 J/m.
 9. The composition of claim 8wherein the ratio of milliequivalents of epoxy to milliequivalents ofhydroxyl is in the range of about 1.5 to 2.5.
 10. A process for forminga three-dimensional article, said process comprising the steps: (1)coating a thin layer of a composition onto a surface; (2) exposing saidthin layer imagewise to actinic radiation to form an imagedcross-section, wherein the radiation is of sufficient intensity to causesubstantial curing of the thin layer in the exposed areas; (3) coating athin layer of the composition onto the previously exposed imagedcross-section; (4) exposing said thin layer from step (3) imagewise toactinic radiation to form an additional imaged cross-section, whereinthe radiation is of sufficient intensity to cause substantial curing ofthe thin layer in the exposed areas and to cause adhesion to thepreviously exposed imaged cross-section; (5) repeating steps (3) and (4)a sufficient number of times in order to build up the three-dimensionalarticle; with the proviso the three-dimensional article has thefollowing properties: (i) a tensile break before yield stress or atensile yield stress greater than 13 N/mm2; (ii) a tensile modulus inthe range of about 180 to about 850 N/mm2; (iii) a tensile breakelongation before yield or a tensile yield elongation greater than 6%;and (iv) a notched Izod impact strength greater than 50 J/m.
 11. Theprocess of claim 10 wherein the composition comprises: (a) about 20-40%by weight of an epoxide-containing material; (b) about 5-40% by weightof an aromatic or cycloaliphatic acrylic material; (c) about 5-50% byweight of a reactive hydroxyl-containing material; (d) at least onecationic photoinitiator; and (e) at least one free-radicalphotoinitiator.
 12. The process of claim 10 wherein the actinicradiation is in the range of about 280-650 nm.
 13. The process of claim10 wherein the exposure energy is in the range of about 10-75 mJ/cm. 14.The process of claim 10 wherein the tensile modulus of the articleproduced is increased by increasing the exposure energy.
 15. The processof claim 14 wherein the article produced has a tensile modulus in therange of from about 275 to about 620 N/mm².